Electrical power generated in either photovoltaic or photogalvanic transducers can be stored in auxiliary systems. For example, the transducers can be used to charge lead acid batteries, to electrolyze water or to drive pumps used to store water in an elevated basin.
The pioneering work of Eugene Rabinowitch accomplished photogalvanic transduction in a thionine-iron cell with two identical platinum electrodes, one illuminated and the other in the dark. Journal of Chemical Physics, Vol. 8, pp 560-566, 1940. A device configuration with greater potential efficiency which also offers the attractive possibility of being developed into a practical area device is the totally-illuminated thin-layer transducer first described by Clark and Eckert, Solar Energy, Vol. 17, pp 147-150, 1975. In such transducers, at least one electrode is selective, undergoing rapid electron exchange with one redox couple while largely blocking electron exchange with the other. In the Clark-Eckert transducer, the dye couple reacts rapidly at an n-type tin oxide anode while electron transfer with iron ions is effectively blocked, presumably because the redox potential of the ferric/ferrous couple falls in the gap between the conduction and valence bands of the semiconductor electrode.
Published studies on photogalvanic conversion have been almost entirely limited to various approaches to transduction such as noted above, but the possibility of intrinsic storage has been recognized. For example, a typical photogalvanic converter depends on the generation of an energy rich photostationary state in an electrolyte and the continuous relaxation of the state toward dark equilibrium by electron transfer through an external circuit. It has been suggested that by inhibiting the relaxation of the photostationary state, power can be stored until relaxation is permitted. Srinivasan et al., The Journal of Chemical Physics, Vol. 52, pp 1165-1168, 1970. Srinivasan et al. suggested the photoreduction of thionine by cobalt (II)-EDTA. The back reaction between cobalt (III)-EDTA and leucothionine was largely prevented by extracting the leuco dye into ether. A significant disadvantage of such a storage system is that the dye is itself the storage element; thus any breakdown of the dye with age results in loss of storage.
Thionine has also been suggested for use in a fuel cell system. Silverman et al., Proceedings of the 14th Annual Power Sources Conference in Fort Monmouth, N.J., pp 72-75, 1960. In that fuel cell system, a dye such as thionine was photoreduced in a regenerator. The leuco dye was then allowed to react at an electrode in a dye regenerating reaction. By means of the photoreduction and regenerating reaction, charged redox states of reagents were obtained in two electrolyte solutions. The solutions were then pumped to a fuel cell and were permitted to undergo a redox reaction in the dark. For recharging, the solution was returned to the illuminated regenerator.
An object of the present invention is to provide an electrical storage cell having structural simplicity near to that of conventional lead acid storage cells yet which can be recharged by direct illumination. A further object of this invention is to provide such a storage cell which does not require that a portion of the cell be retained in a dark environment. Yet another object of this invention is to provide such a cell that does not require that the photostationary state of a photoredox couple be maintained to retain the cell in the charged state.